Thanks for replying.
The problem is that if the pen is put together without a ballpoint refill in, then the cap can not be unscrewed as it is the refill which stops the threads in the cap from spinning.
I have 6 or 7 other carene BP and was lucky enough to be able to take one apart and figure out how it works.
I fixed it with a length of fine stainless steel wire (motorcycle lock wire), doubles up to provide a cam surface, and put in through the hole in the end where the ballpoint pokes out, to stop the mechanism spinning. I was then able to unscrew the cap (with a bit of mucking about to get the wire to lock against the cam in the cap) and return it to service.
Now with a refill put in it now works perfectly and can be easily taken apart to put a new refill in.
The moral of the story is I guess, don't put a carene ballpoint together without a refill. I never would have guessed that no refill would cause this problem.